The invention relates to a sensing device which can be applied upon the surface of a road, flush with the surface, for sensing the passage of vehicles over the device and, if connected with a properly programmed computer, determining the weight of the vehicle passing over the device.
Road sensing devices that are commonly used generally comprise an elongated tube or hose which is laid across a road so that passing vehicles run over it. Typically, the pressure of the vehicle tires causes the tube or hose to compress momentarily which affects the air pressure within the tube or hose. By use of a suitable air pressure responsive indicator, the presence of vehicles can be recorded. Such detectors are relatively easily damaged in use. In addition, they are not capable of producing sophisticated indications of location of the vehicle wheel on the road, vehicle weight indications and the like.
In addition, a cable like, piezo-type axle sensor has been used. Generally, this consists of a central, or inner conductor which is surrounded by a piezo ceramic material, in turn surrounded by an outer tubular conductor. Pressure on the cable-like sensor causes an electrical signal to flow between the conductors, which signal may be proportional to the amount of pressure. However, this sensor is prone to false signals because the round cable is susceptible to pressure from any direction, including pressure from pavement movements, heavy weights and poor truck suspension systems. Also, it functions poorly under light pressure from light vehicles and from slow speed vehicles, since piezo material is a rate of change or speed dependent material.
Recently, compression or force sensitive resistors have become available. These resistors or semi-conductors normally resist the flow of electrical current, but permit the flow in proportion to pressure applied to the resistor. That is, by squeezing or compressing the resistor, it becomes less resistant to the flow of current so that the flow of current can be measured by a suitable detector that indicates the fact of the application of pressure as well as the amount of pressure and, also, the location of the pressure upon a particular resistor. By way of an example, a force sensitive resistor may comprise an elongated, printed circuit strip having electrically conductive stripes printed upon a substrate, with a repetitive pattern of gap-like areas between the stripes. These areas may be covered by a compression responsive semi-conductor film or coating applied upon a non-conductive plastic substrate. The film or coating may be formed of conductive, metallic micron size particles contained as a matrix, within a suitable non-conductive plastic material. Upon the application of pressure to the film, the resistance to electrical flow through the film decreases or, alternatively, the amount of electrical contact between the film and the conductive stripes increases, so that the film or coating may serve as an electrical shunt across the particular gap area which it overlaps. Consequently, pressure applied upon the device results in current flow through the printed circuit across the gap area beneath the pressure. The amount of pressure and the location of the pressure along the resistor printed circuit can then be detected.
Examples of such compression sensitive devices are disclosed in U.S. Pat. No. 4,314,227 issued Feb. 2, 1982, U.S. Pat. No. 4,315,238 issued Feb. 9, 1982, U.S. Pat. No. 4,451,714 issued May 29, 1984 and U.S. Pat. No. 4,489,302. These patents disclose switch-like constructions including a film of fine particles of molybdenum disulfide which are formed into a film upon a nonconductive sheet to provide microprotrusions that are in intimate contact with separated conductive sections of a conductive material. Upon application of pressure the contact increases so that current flows through the shunt-like film. An alternative film is made of conductive particles that are loosely packed together, so that the loose mass is essentially not conductive, and which compress together under pressure to make good, conductive contact so as to shunt current between the separated conductive sections.
The foregoing type of force sensing resistor is relatively fragile and sensitive. Thus, such resistors would not ordinarily be considered suitable for use in a rugged, relatively destructive environment. However, the invention of this application is concerned with a sensing device, which utilizes a force sensing resistor, that may be used in the outdoor, highly destructive environment of a road for producing accurate, and repeatable indications of vehicle passage, vehicle weight, and the like useful information.